In order to impart a flame-retardancy to a polyolefin composition which has been often used in coating wires, cables and the like; hoses; sheets; injection moldings; and the like, a halogen compound and diantimony trioxide have heretofore been added to the polyolefin to achieve the flame-retardancy. However, these compositions are halogen-containing compositions, so that halogen gases, which are not only harmful in themselves but also the cause of metal corrosion, are generated when they are burned. Therefore, these compositions have not been desired. In addition, the amount of smoke generated is large, the visibility becomes bad and the people have been markedly prevented from taking refuge from a fire and from fighting a fire.
In particular, from a safety aspect, it has recently been strongly required that such a halogen-containing gas be not generated. Under such circumstances, attention has been paid to an inorganic flame-retardant using a hydrated metal compound which generates very little fumes and is not substantially harmful.
In recent years, a resin composite containing magnesium hydroxide, aluminum hydroxide or the like has been put to practical use for imparting flame-retardancy; however, these flame-retardant resin compositions can prevent the generation of halogen-containing gases but have a problem in that there is caused a so-called drip phenomenon that the resin compositions melt and drip during the firing or in that the resin compositions are inferior in shape-retainability at high temperatures.
In order to improve the flame-retardancy and also the heat resistance to solve the above problem, there is a method of crosslinking the resin, and this is disclosed in, for example, JP-B-57-24373, JP-B-57-26620 and the like. However, the method described therein requires a large scale crosslinking apparatus for chemical crosslinking, crosslinking with electron beams or the like and hence the cost of the equipment per se and the expenses for subsequent operation, maintenance, control and the like of the apparatus are increased, resulting in an increase of the cost of the composition. Furthermore, as stated in JP-A-60-101129 and JP-A-60-147463, in the case of a flame-retardant, crosslinked composition characterized in that a flame-retardant is added to a silane-grafted polyolefin resin and this silane-grafted polyolefin resin is finally crosslinked, only the silane-grafted polyolefin resin whose crosslinking has been controlled has been able to be used for preventing premature crosslinking in blending and kneading the flame-retardant with the silane-grafted polyolefin resin. As a result, it has been impossible to increase the degree of crosslinking of the silane-grafted polyolefin resin and hence the heat resistance has been insufficient.
The above method requires at least two reaction steps, namely, the silane-grafting reaction step and the silanol-condensing reaction step. Accordingly, at least two extrusion steps must be carried out and an economical difficulty of the final product is unavoidable.
Furthermore, the Monosil TM process method is known as a one-step process. However, this method requires a liquid-adding means for injecting an organic unsaturated silane in the form of a liquid into an extruder and has problems of slippage and metering failure. The extruder is also required to have a large L/D and be of an expensive and special type for uniformly dispersing a small amount of an additive, and hence, an economical problem is unavoidable. In addition, a very sophisticated technique is necessary for the extrusion and hence there have been no commercialized method for flame-retardant type.
In addition, JP-A-3-167229 discloses, as a one-step process, a silane-crosslinking method in which a silane has been introduced into a solid carrier polymer. However, this method uses a porous polymer or EVA as the solid carrier polymer, and in this method, in addition to the silane and the free-radical generator, an additive such as a silanol-condensing catalyst, an antioxidant or the like is also introduced into the solid carrier polymer. Therefore, an oligomer is formed by condensation of the silane or the crosslinking is inhibited by trapping the radical, whereby such problems are caused that crosslinking efficiency and storability are inferior. In said Japanese publication, no flame-retardant type is mentioned.
This invention has solved these problems and aims at providing a silane-crosslinking method comprising silane-crosslinking a flame-retardant polyolefin containing a silanol-condensing catalyst and a flame-retardant, wherein a flame-retardant, silane-crosslinked polyolefin is produced in one step using a carrier polymer containing an organic unsaturated silane or the like at a high concentration.